The present invention relates to polymeric protective coatings for optical fibers, and more particularly to improved radiation-curable acrylate optical fiber coatings offering excellent moisture resistance and stability.
Glass optical fibers are a relatively recent innovation in the field of telecommunications. As is well known, protective coatings are customarily applied to these fibers at the time of manufacture, both to preserve the inherent strength of the fibers and to protect them from certain types of bending which can induce signal loss in telecommunication cables.
These requirements dictate that the coating applied to the optical fiber have both substantial toughness and yet be soft enough to distribute transverse strain applied to the fiber. An additional requirement for optical fiber coating materials derives from the fact that very high optical fiber drawing speeds are now being employed in the industry for reasons of manufacturing efficiency. For economic production, therefore, only coating materials which can be rapidly applied to and cured on the surface of the optical fiber are useful.
Currently, the preferred coating materials for rapid optic fiber production are radiation-curable coating formulations which can be very rapidly cured by UV irradiation. One family of particularly preferred coatings of this type comprises the radiation-curable polyurethane or polyurethane-polyurea acrylate materials. These are typically ultraviolet-curable formulations comprising acrylated polyurethane oligomers, available in liquid form, which can be cured to films which exhibit good softness over a very broad temperature range, good tensile strength and toughness, and rapid UV curing characteristics.
Specific examples of UV-curable acrylate compositions such as described are reported in published European patent applications EP 0204160 and EP 0204161. The compositions disclosed in these applications are based on resins more specifically designated acrylate-terminated polyurethane, polyurea, or polyurethane/polyurea oligomers.
It is of course important that the resin formulations selected for application to these optical fibers exhibit excellent resistance to moisture penetration and/or chemical breakdown over an extended period of time. Thus minimal interaction with water or water vapor and a polymer structure which resists chemical breakdown over a wide range of ambient conditions for the entire anticipated service life of the coated fiber are considered essential requisites of these coatings.
Existing urethane acrylate coating systems can be judged to exhibit excessively high water absorption for use in environments high in moisture content. Water absorption in these environments can be a critical problem since it can affect coating geometry and thereby cause increased fiber attenuation over time.
Notwithstanding the recognition of this problem, changes in acrylate coating composition to solve the problem have to date not been universally accepted in the industry. This is due in part to the fact that any changes in coating formulation which result in significant reductions in curing speed, or which degrade the chemical stability of the cured coating, are not commercially acceptable.
Cured coating stability is a factor of increasing importance in the formulation of new coating compositions. K. Ohashi et. al., in "Mechanisms of Hydrogen Evolution and Stabilization of UV-Cured Urethane Acrylate Resin for Coating of Optical Fiber", Polymer Degradation and Stability, 22 (1988) 223-232, review the problem of hydrogen evolution in cured coatings and some approaches to the solution of those problems. Y. Ohashi et. al., in Chemical Abstracts, 110 (4):25492q, disclose the utility of phosphite compounds for the suppression of hydrogen generation in polyether-based urethane acrylates, although a great many other formulation changes have of course also been proposed. Again, however, these proposed changes in composition for improved coating stability are rarely commercially successful, unless the "improved" composition also exhibits curing speed and moisture resistance which are at least equivalent to the coatings currently in commercial use.
It is therefore a principal object of the present invention to provide improved UV-curable acrylate coating formulations offering superior resistance to moisture attack, excellent curing speed, and excellent long-term chemical stability.
It is a further object of the invention to provide a coated optical fiber incorporating an improved urethane acrylate coating which offers better lifetime optical transmission characteristics in both moist and dry environments than prior art optical fibers.
Other objects and advantages of the invention will become apparent from the following description thereof.